1. Field of the Invention
The present invention relates to a control device of a power transmission device which drives a joint and the like of a robot.
2. Description of the Related Art
Conventionally, in an industrial robot and the like, a position control (a servo control) is generally performed, in order to drive a joint as a power transmission device between a primary element (a driving element) and a secondary element (a driven element). Further, as a control method for the position control, a technique which adopts a sliding-mode control, for example, in Japanese Patent Application Laid-Open No. H5-134758 (hereinafter referred to as Patent Document 1) is proposed. In this technique, a switching function used for the sliding-mode control is determined by a format of proportional integral control.
In the position control of the power transmission device such as the joint, the position of the secondary element (the driven element) may be controlled precisely to a desired position. However it lacks flexibility under various external environments in which a position or a shape of an external object contacting the secondary element, or a disturbance and the like, is difficult to specify or predict beforehand. For example, in a case where the secondary element contacts an unpredicted external object and the like, situations where it becomes difficult to appropriately move the secondary element, or situations where an excessive external force acts on the secondary element, tends to occur.
Therefore, in recent years, in order to realize a robot and the like capable of operating flexibly under various external environments, a power transmission device of a structure in which the primary element and the secondary element are coupled by a member capable of deforming elastically (hereinafter sometimes referred to as an elastic deformation member), such as a spring member, and a force applied to the secondary element via the elastic deformation member is controlled to a desired value, has been studied by the present inventors and the like.
In an operational control (a power control) of the power transmission device of a structure in which the primary element and the secondary element are coupled via the elastic deformation member, it is generally difficult to perform stable control in which oscillation and the like of a control system does not occur with respect to various condition variation such as a variation in an inertia of a load, by a versatile control method such as a PD control.
Therefore, the present inventors had attempted to adopt a method of a sliding-mode control, which has a characteristics of having high robustness with respect to variation of disturbance and the like, in the operational control of the above-mentioned power transmission device including the elastic deformation member.
The sliding-mode control is for converging a state amount of a control object to a desired value, on a switching hyperplane that is defined by a switching function (a hyperplane represented by a format of a switching function=0). Therefore, in the sliding-mode control, there is a necessity of appropriately setting the switching hyperplane.
The term “hyperplane” is an expression of generalizing a plane in a phase space of a plurality of dimensions. The hyperplane means a straight line in the phase space of two dimensions, and a normal plane in the phase space of three dimensions.
As a method of setting the switching hyperplane to be used in the sliding-mode control (specifically, a method of determining the coefficient of the switching function), a method of determining the coefficient of the switching function so as to minimize a predetermined evaluation function by applying, for example, a method of an optimum control, or a method of determining the switching function in a manner as is shown in Patent Document 1, are conceivable.
However, in a case of applying the method of the optimum control in order to determine the coefficient of the switching function, there is a necessity of appropriately determining a value of a weight coefficient in the evaluation function. Further, even in the technique disclosed in Patent Document 1, there is a necessity of appropriately determining a value of a gain related to each member of the switching function.
A current status is that, in performing the operational control of the power transmission device performing the power transmission between the primary element and the secondary element through the intermediary of the elastic deformation member by the sliding-mode control, a guideline on how to determine the value of the weight coefficient of the evaluation function or the value of each gain in Patent Document 1, or a guideline on efficiently performing such determination, has not been established yet.
Therefore, in the method of determining the coefficient of the switching function so as to minimize the predetermined evaluation function, or in the method of determining the switching function in the manner as is shown in Patent Document 1, numerous trial and error must be repeated in order to determine the value of the weight coefficient in the evaluation function or to determined the value of each gain in the technique of Patent Document 1.
Consequently, the gradient of the switching hyperplane set in advance by the above mentioned methods may become inappropriate in some operational situation of the power transmission device, and there is a possibility of losing robustness of the control in such operational situation of the power transmission device.